1. Field of the Invention
The invention relates to wireless network elements and, more particularly, to a network element for implementing scheduled high-power Point To Point (PTP) and low-power Point To MultiPoint (PTMP) transmissions.
2. Description of the Related Art
Wireless data communication networks may include base stations, relay stations, subscriber stations, and other network devices, interconnected and configured to handle data as it passes through the network. These devices will be referred to herein as “network elements.” In a wireless network, these network elements are interconnected by transmitting wireless signals in a portion of the electromagnetic spectrum.
When signals are transmitted wirelessly, the distance at which an RF signal may be received is often directly related to the amount of power used to transmit the signal and the directionality of the antenna. Accordingly, the amount of power that may be used to transmit data in the wireless spectrum is regulated in many jurisdictions. For example, in the United States, significantly more power may be used to transmit data for Point to Point (PTP) transmissions, which are limited to a maximum of up to 200 W EIRP in the downlink direction, than can be used for Point To MultiPoint (TPMP) transmissions, which are limited to a maximum of 4 W EIRP in the downlink direction. Limitations on the amount of power help prevent interference between adjacent regions in the wireless network and between wireless networks operated by different companies.
One emerging wireless protocol that may be used to transmit data between network elements is specified in Institute of Electrical and Electronics Engineers (IEEE) standard 802.16. The 802.16 standard can be used to create PTP or PTMP links with channel sizes that range from about 1.25 to 20 MHz, which enables the WiMax implementation of the 802.16 standard to provide T1 and higher data rates. 802.16 specifies three different physical layers—256-point FFT Orthogonal Frequency Division Multiplexing (OFDM), single carrier, and 2048 Orthogonal Frequency Division Multiple Access (OFDMA) modes. Of these three, only 256 OFDM has been accepted globally and, hence, is the preferred physical layer. One embodiment of the 802.16 standard that uses this accepted interface is commonly referred to as WiMax (Wireless Interoperability for Microwave Access). Although the term WiMax is used commonly to refer to an implementation of the 802.16 standard that uses the accepted physical layer, the term WiMax will be used more expansively in this document to refer to any implementation of the 802.16 standard.
The 802.16 standard divides time into frames, with each frame containing a downlink subframe and an uplink subframe. FIG. 1 shows graphically the frame structure set forth in the 802.16 protocol. Specifically, the frame 200 includes a downlink subframe 210 and an uplink subframe 220. The downlink subframe includes an header 230 containing information that will allow the subscriber stations to synchronize with the base station. For example, the header 230 contains an idle interval 231, a preamble 232 formed of two symbols, a Frame Control Header (FCH) 234 formed of two symbols, and both a downlink map 236 and an uplink map 238.
The downlink and uplink subframes are divided into time slots referred to as bursts that may be assigned to the subscriber stations using a number of different scheduling mechanisms. Generally, a base station will perform centralized scheduling although the standard also supports decentralized scheduling when the network elements are used to implement a mesh network topography. Once the burst are scheduled (either centrally or in a decentralized manner), the subscriber stations will be notified of their allocated downlink burst(s) in the DL map 236 and will be notified of their allocated uplink burst(s) in the UL map 238. Additional details associated with the physical and Media Access Control specification may be found in the 802.16 specification, the content of which is hereby incorporated herein by reference. To enable a subscriber station to communicate with the base station, the subscriber station will listen to the carrier frequency during its allotted DL burst, and will transmit on the carrier frequency during its allotted UL burst.
Within a time-slot (or link to a particular subscriber) WiMax enables the transmission characteristics to be optimized for that link. For example, WiMax enables each subscriber's data rate to be optimized by allowing the base station to set modulation schemes on a link-by-link basis. A subscriber close to the base could use 64QAM (Quadrature Amplitude Modulation), while a weaker signal from a more remote subscriber might use a different modulation scheme such as 16QAM or Quadrature Phase Shift Keying (QPSK). These choices may be made for both the uplink direction (from the subscriber to the base station) and downlink direction (from the base station to the subscriber).
Additionally, WiMax allows the power level to be adjusted on a link-by-link basis so that the amount of power required to be transmitted by the subscriber stations in an uplink direction may be minimized. Power control, in this instance, relates to the amount of power used by the subscriber station, and is implemented by causing the base station to send power control information to each of the subscriber stations to allow the subscriber station to use the least amount of power required to transmit data to the base station.
Often it would be advantageous to connect the base station with the core network using wireless transmissions rather than a physical link. For example, deploying a copper or optical cable is relatively difficult and, hence expensive, as compared to the relative ease with which a wireless link may be established interconnect the base station with the core network. Additionally, when the base station is mobile, base station it is not possible to use fixed wireline access to provide a backhaul connection between the base station and the fixed wireline network. Accordingly, it would be advantageous to provide a network element that would be able to operate simultaneously on the backhaul and the access portions of a wireless communication network.